The present invention relates to non-ionic low diffusing photo-acid generators (PAGs) for lithographic applications, and more specifically to PAGs comprising sulfonate esters of fluoroalkyl substituted alpha-hydroxy aryl ketones.
Extreme Ultraviolet (EUV) lithography is expected to succeed current 193 nm immersion lithography combined with multiple patterning enhancements as the next generation printing technique. EUV radiation, with a shorter wavelength of 13.5 nm, is expected to achieve sub-20 nm features in a single exposure process. However, more advances in efficient light sources, EUV masks, and resists are needed for EUV lithography to become a manufacturing process.
During the last few years, considerable effort has gone into the development of resists for EUV applications. However, the majority of the EUV resists have been modified from the resists developed for 193 nm and 248 nm applications.
The highest performing photoresists for 193 nm and 248 nm applications are all based on a chemical amplification mechanism. Chemically amplified photoresists utilize a catalytic mechanism to generate a relatively large number of chemical events (e.g., deprotection reactions in the case of positive tone photoresists, or crosslinking reactions in the case of negative tone photoresists). Application of a relatively low dose of radiation induces formation of the catalyst, often a strong acid, which then catalyzes the chemical events. The current positive resist compositions comprise aqueous base soluble functional groups that are sufficiently protected with acid-labile groups so that the resist initially will not dissolve in an aqueous base developer. During exposure to radiation, the photo-acid generator (PAG) present in the resist composition produces a strong acid, which then catalyzes the removal of the acid-labile groups upon heating exposed resist layer in a post-exposure bake (PEB). This process produces aqueous base soluble material in the exposed area, which then is selectively removed with a basic aqueous developer to produce the images.
One phenomenon that limits the resolution potential of the resists developed for 248 nm, 193 nm and E-beam applications is referred to as “image blur” (see, e.g., Hinsberg et al., Proc. SPIE, (2000), 3999, 148). Image blur is generally thought to result from two contributing factors: gradient-driven acid diffusion and reaction propagation, the result being a distortion in the developable image compared to the projected aerial image transferred onto the film. This becomes critical in EUV applications because of the need for small features with low line edge roughness (LER). Therefore, a need exists to control the gradient driven acid-diffusion in the resist films.
Most widely reported PAGs in the resist formulations are ionic in nature (triphenylsulfonium or iodonium sulfonates). Non-ionic PAGs have some advantages such as higher solubility in casting solvents and homogeneous distribution in the resist film. Previously, a few non-ionic PAGs having imide photo-labile groups have been reported (U.S. Pat. No. 8,329,377 B2 to Takemoto, et al.).
In the area of photo acid generators (PAGs), a limited number of PAGs having aryl ketone protecting groups have been reported. U.S. Pat. No. 5,304,456 to Ueda, et al., discloses PAGs with perfluoro alkyl sulfonic acids. WO02/082185/A1 (JP2004519520A) to Ferreira, et al., and JP2002236358A (JP4145017B2) to Kunihiko disclose PAGs with perfluoroalkyl and perfluoro ether substituted sulfonic acids. Similarly, aryl ketone triflate PAGs have been described by Storer, et al., Analytica Chimica Acta (2006), volume 558(1-2), pages 319-325. These previously reported aryl ketone PAGs produce volatile and highly diffusing sulfonic acids that can be unstable in resist formulations.
Therefore, a need exists for aryl ketone protected PAGs that have higher thermal and hydrolytic stability in resist formulations and produce less volatile, low diffusing sulfonic acids.